Using a quadrupole ion-trap mass spectrometer with an electrospray ionization source, the Cooks kinetic method has been used to measure the lithium and sodium ion binding energies of the N-acetyl and N-glycyl derivatives of a series of 14 amino acids. For comparison, the gas-phase basicities of the amino acid derivatives were also determined by the kinetic method. The lithium binding free energies range from 47.2 to 56.4 kcal/mol, and the sodium affinities from 30.8 to 41.2 kcal/mol. Comparisons between basicities and metal ion binding energies indicate that the presence of a coordinating group (e.g., -OH, -CO2H, etc.) in the amino-acid side chain can significantly increase the lithium and sodium binding energies. Dynamics calculations (CHARMm) confirm that side-chain coordination is a common stabilizing effect in the metalated systems. A good correlation, with a slope near unity, is found between the metal ion binding energies of the N-acetyl and N-glycyl derivatives. This indicates that the two groups of compounds are adopting similar coordination schemes and strongly suggests that zwitterionic forms of the peptide derivatives are not important.
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